Elastic leaf seals have been used for many years to control air leakage between movable surfaces. In particular, they have been used in aircraft engine exhaust nozzles where high internal pressure is utilized to push the leaf seal into a sealing position against an adjacent seal surface. One conventional elastic leaf seal design comprises two overlapping metallic members fabricated from thin sheet metal. The members may be slotted or otherwise bent to conform the seal to conical or irregular surfaces and to accommodate thermal growth at the ends of the seal. Since these seals are sometimes susceptible to inelastic buckling from induced thermal stress, slots or gaps are generally used to prevent warping of the seal.
The effectiveness of the seal is primarily dependent upon the initial gap, the flexibility of the seal and the differential pressure across the seal. The thickness of the seal must be adequate to support the pressure differential across it but thin enough to deflect and close a gap between adjacent parts.
If the seal is made too thin, pressure upon the seal may exceed the yield strength of the seal material and result in seal breakage or blow out causing a loss of sealing between the adjacent parts. If the seals are made thicker, exhaust gas pressure may be insufficient to deflect the seal material and to produce a good seal.
To meet these two conflicting requirements the seal thickness is usually held between 0.007 and 0.010 inches thick in jet engine exhaust nozzles. These thin leaf seals are therefore extremely vulnerable to damage during assembly or operation of the exhaust nozzle.
Another problem that arises with elastic leaf seals is due to manufacturing difficulties. When these seals are used for aircraft engine or spacecraft engine nozzles some seals are relatively long. In aircraft engine nozzles 2-4 foot long leaf seals are not uncommon. Since the seals are made from very thin leaves of sheet metal it is very difficult to hold tolerances between the leaf seal and the part to which it seals. Variations in part tolerances result in large variations in the gap between the seal and its adjacent part. Although a gap of 0.002 to 0.003 inches may be desired, normal variations results in seal gaps as large or larger than 0.050 inches at some location along the length (or circumference) of the seal.
Large gaps may result in the leaf seal failing to move into its preferred sealing position due to excess exhaust gas leakage causing the seal to flutter. Exhaust gases would therefore continue to escape from the nozzle resulting in a loss of directed thrust for the aircraft it is associated with. In some instances the loss of exhaust gases can result in thermal damage to the surrounding nozzle structure due to high temperature exhaust impinging upon insufficiently cooled structures.
In view of the above a need exists for an improved air seal for high temperature and high stress environments.
It is an object of this invention, therefore, to provide a seal capable of withstanding the harsh conditions of an aircraft engine exhaust environment.
It is further object of this invention to provide a seal capable of sealing between movable parts without reliance on a pressure activated sealing arrangement.